I. Field of the Invention
This invention relates to processes for treating BOD-containing wastewaters, such a municipal sewage, industrial wastewaters and the like and, more particularly, to such processes which are capable of removing both BOD and nutrients, i.e., phosphorous and/or nitrogen, from wastewaters.
II. Prior Art
Activated sludge processes have been used for some time to remove biological oxygen demand (BOD) from municipal sewage, industrial wastewaters and the like. In such a process, a wastewater influent is mixed with a microorganism-containing recycled biomass or activated sludge in an initial contact zone to form a mixed liquor. At some point in the process, the mixed liquor is aerated with sufficient oxygen to grow and maintain a satisfactory population of microorganisms which sorb, assimilate and metabolize the BOD of the wastewater.
Spector U.S. Pat. No. Re. 32,429 discloses an activated sludge process for biologically removing phosphorous. In this process the wastewater and recycled activated sludge are mixed in an initial contact zone from which air is excluded and the mixed liquor is subsequently treated in one or more aeration or aerobic zones before being clarified to separate an activated sludge. The initial contact zone is maintained in the absence of externally supplied oxygen and under conditions where the nitrite and/or nitrate concentration (NO.sub.x --) is less than 0.3 ppm, preferably less than 0.2 ppm, and the dissolved oxygen content (DO) supplied by the wastewater influent and recycled sludge is less than 0.7 ppm, preferably less than 0.4 ppm. Such low DO levels in the anaerobic zone are achieved by introducing nitrogen gas to strip the mixed liquid of any dissolved oxygen and prevent entry of oxygen from the atmosphere. The NO.sub.x -- concentration in the anaerobic zone is maintained at such levels by avoiding introduction of mixed liquor from an aerobic zone and controlling the NO.sub.x -- concentration in the recycled sludge. To provide denitrification, three treatment zones are used, an above-described anaerobic zone, an anoxic zone in which the DO is in excess of 0.7 ppm, the NO.sub.x-- concentration is in excess of 2 ppm and the nitrites and/or nitrates are reduced to nitrogen gas and an aerobic zone in which oxygen is introduced to oxidize BOD and effect phosphate uptake. A portion of the mixed liquor from the aerobic zone is recycled to the anoxic zone to maintain the NO.sub.x-- concentration in the anoxic zone at the desired level.
Another type secondary treatment process employs so-called rotating biological contactor (RBC) units in the form of a plurality of plastic disks which are relatively densely packed on a shaft but spaced apart to form a cylinder. The disks provide surfaces for the growth of a biomass. The RBC units typically are located to be rotated partially submerged in a treatment tank containing wastewater so that the surfaces are alternately exposed to the wastewater and to oxygen in the ambient air. A film of wastewater is carried into the air and trickles down the surfaces of the RBC's while absorbing oxygen from the air. Organisms in the biomass remove dissolved oxygen and organic materials from the film of wastewater and unused dissolved oxygen in the film is mixed with the contents of the mixed liquor in the treatment tank. Such systems typically employ a single pass flow and phosphorous removal is accomplished by adding a chemical to the last RBC stage. Some single pass systems employ a carbon source, such as methanol, for nitrogen removal and a chemical coagulant for phosphorous removal.
Lorpey U.S. Pat. Nos. 3,817,857 and 3,869,380 disclose processes for removing carbonaceous and nitrogenous materials from wastewaters including a single stage treatment unit employing RBC units and a denitrifying unit employing RBC units located upstream of the treatment unit. The denitrification unit is supplied wastewater and recirculated effluent from the treatment unit. There is no recirculation of sludge from the final clarifier to the treatment unit. Such processes do not remove phosphorous effectively.
Lorpey U.S. Pat. No. 3,871,999 discloses a single pass, two stage process employing RBC units in both stages. The first stage is operated to remove most of the BOD and convert a small portion (less than 50%) of the ammonia nitrogen into nitrates and nitrites and produce an effluent containing nitrogen in the form of nitrates and ammonia nitrogen. The second stage is operated with the exclusion of molecular oxygen to provide an oxygen deprived atmosphere which forces a biochemical conversion of substantially all the remaining ammonia nitrogen into nitrogen gas. This process does not employ recycled sludge and does not remove phosphorous effectively.
Lorpey U.S. Pat. No. 3,915,854 discloses that existing activating sludge processes can be upgraded by adding a single stage biological treatment step employing RBC units between an activated sludge treatment tank and the final clarifier, thereby eliminating the need to recycle activated sludge from the final clarifier to the activated sludge treatment tank.
Guarino U.S Pat. No. 4,093,539 discloses that the BOD removal efficiency of an activated sludge system can be improved by adding partially submerged RBC units to the aeration tanks. The process otherwise is operated as a conventional recycled activated sludge process which typically has a hydraulic retention time of 6-12 hours. This patent is concerned primarily with BOD removal and ammonia conversion (nitrification) and does not specifically address removal of nitrites and/or nitrates (denitrification) or removal of phosphorous.
Ankaitis U.S. Pat. No. 4,721,570 discloses that the settleability of suspended wastewater solids in a treatment system employing RBC units in a series of separate compartments in an elongated treatment tank can be enhanced by maintaining the last compartment at the effluent end of the tank at a DO of at least 0.5 mg/l and recycling sludge from the final clarifier to the effluent end of the tank, instead of the influent end of the tank. This patent does not address removal of either phosphates or nitrogen.
Applicant is unaware of any prior RBC wastewater treatment process which is capable of also biologically removing phosphorous or both phosphorous and nitrogen. Such a process is particularly desirable for existing RBC installations subject to governmental agency regulations requiring reduced phosphorus or phosphorous and nitrogen concentrations in the final effluent.